Fig. 5.

21-year-old man presenting with several-week-long history of painful mass in his left thigh that was “growing by day.” Patient denied history of trauma, and radiography results were negative.

A, MRI shows nonspecific soft-tissue mass in posterolateral thigh with diffuse enhancement and surrounding edema, as seen on fat-saturated axial T1-weighted image obtained after administration of gadolinium contrast agent.

B, Follow-up CT suggests final diagnosis of heterotopic ossification (myositis ossificans) by revealing characteristic peripheral zone (“eggshell”) of ossification (arrow). Patients with myositis ossificans are frequently young patients without known history of trauma, and thigh is most common location. Rim of mineralization is typically seen on CT by 4–6 weeks and is important discriminator from soft-tissue tumor.